A Twitter account linked to the Islamic State, the extremist group also known as ISIS or ISIL, called out specific assassination targets in the U.S. for the first time Monday — specifically, Twitter employees. But the social media company isn't freaking out just yet.

"The time has arrived to respond to Twitter’s management by directly attacking their employees and physically assassinating them," read one of several threatening tweets from an account named @Dawlamoon. It added that the attack would be carried out "by the sleeper cells of death."

A later tweet stated: "Every Twitter employee in San Francisco in the United States should bear in mind and watch over himself because on his doorstep there might be a lone wolf assassin waiting." All the tweets were written in Arabic, accompanied by a hashtag that translates to "the concept of lone wolf attacks."

The @Dawlamoon account is linked to a Jerusalem-based group called Al Nusra Al Maqdisia, which is linked to and frequently mentioned in Islamic State forums, according to a report from Vocativ. Its exact connection to the Islamic State remains unknown.

Twitter suspended the @Dawlamoon account, which had more than 3,000 followers, after the death threats were issued Monday, and opened an official investigation.

"Our security team is investigating the veracity of these threats with relevant law enforcement officials,” a Twitter spokesperson told Mashable.

Twitter has been shutting down ISIS-related accounts repeatedly over the last few months, but new ones are continually cropping up. The group has also been using the social network Diaspora, initially established as a rival to Facebook. A fugitive American citizen named Ahmad Abousamra is wanted in connection with the group's social media efforts.

Twitter would not say whether security at its headquarters had been beefed up as a result of the threats.

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